The present invention relates to a method of producing a semiconductor laser element and, in particular, to suppression of mirror degradation of the semiconductor laser element.
A semiconductor laser element comprises a laser resonator with opposite facet mirrors. The resonator comprises a layered structure including cladding layers and an active layer provided between the cladding layers, all of which layers are made of semiconductors. The resonator is provided with electrode contact layers on the upper most and lowermost layers of the layered structure. A light is excited in the active layer and emitted therefrom through one of the end faces of the resonator.
In a high power laser of the type described, it is known in the art that the laser suffers from mirror degradation to reduce its life time. In detail, the laser light is absorbed at end faces of the resonator due to presence of surface levels to cause a local heating which causes optical damage, or catastrophic optical damage (COD), of the end faces of the resonator. The absorption of light is increased by an occurrence of oxidation and crystal defects. Therefore, the local heating results into the mirror degradation of the resonator. It is needless to say that an increased laser output power causes considerable rise of temperature at the end faces of the resonator.
In order to suppress the local heating by the laser light absorption, there have been known in the art various window structures each being formed around opposite end faces of the resonator. The window structure comprises a window layer formed at the end of the active layer. The window layer is an epitaxially grown layer of a semiconductor material with a forbidden band wider than that of the active layer.
It is disclosed in IEEE Journal of Quantum Electronics, vol. 25, No.6, June 1989, pages 1495 to 1499 (Reference I), that the active layer is removed from the regions around both facets of the large optical cavity together with the upper and lower layers thereof. Then, in the removed regions, an upper cladding layer is regrown to form the window layer.
The arrangement of Reference I has problems that the laser element is complex in its structure and in the window forming process which comprises steps of patterning, selective etching, and regrowing. In the window forming process of the semiconductor laser which uses semiconductor materials containing Al (aluminum), a strong oxide film is formed at an interface with the regrown layer whereby the window structure is accompanied with many surface levels. This is because the active layer is exposed to the air after the selective etching before the regrowing. Since presence of the surface levels increases the light absorption, the catastrophic optical damage (COD) is not expected to be well suppressed.
Unexamined Japanese Patent Application Publication No. Hei 5-13878 (JP-A 5-13878) (Reference II) discloses a semiconductor laser element where a window layer is provided between each of the end faces of the resonator and an end protective layer. The window layer is of a semiconductor material having a forbidden band larger than that of the active layer of the resonator in the band width and has a thickness to prevent occurrence of any local lattice defect caused by lattice mismatching rate or lattice inconsistency between the semiconductor materials of the window layer and the active layer at the interface therebetween. In practice, the active layer and the window layer are described made of Al.sub.x Ga.sub.1-x As and Al.sub.z Ga.sub.1-z As, respectively. In producing processes of the semiconductor laser element using the Al-containing semiconductor materials, the end faces of the resonator cleaved in the air are accompanied by oxide layers which are formed by natural oxidization during the cleaving step in the air. The oxide layers are too strong so that they are difficult to be perfectly removed before growing the window layers. This makes it difficult to form the window layers without any surface level.
Further, in Reference II, electrode layers are formed after the cleavage followed by the formation of the window layers on the cleaved end faces. This means that the laser element according to Reference II has another problem that formation of electrode layers is a difficult operation because the electrode materials come into contact with the active layer.
U.S. Pat. No. 5,063,173 or No. 5,144,634 (Reference III) discloses a semiconductor laser element wherein end faces of the resonator formed by cleaving is coated by amorphous layer of Si, Ge, or Sb, as a passivation layer, to lower the degradation of the semiconductor laser element. However, Si, Ge or Sb is readily diffused into the semiconductor materials such as Al--Ga--As of the active layer and other layers of the semiconductor laser resonator, so that the laser resonator is degraded in its performance by the diffusion.